At approximately what age does calcification (initial mineralization) of the permanent second molars typically begin, according to standard tooth development charts?

Per standard tooth chronology charts (e.g., Logan and Kronfeld), calcification of the permanent second molars typically begins around 2.5 to 3 years of age — earlier than many students initially expect, since the second molars don't actually erupt until around age 11-13. (Permanent first molar calcification begins at birth; third molar calcification begins later, around age 7-10.)